1. Field of Disclosure
The present disclosure relates to an improved method of and apparatus for extending the depth of focus (DOF) of a laser beam used in a laser scanning bar code symbol reading system, and more particularly, extending the depth of focus of the laser scanning beam over both the far-field and near-field portions of the laser scanning field.
2. Brief Overview of the State of The Art
The use of laser scanning bar code symbol reading systems is well known in the art. As shown in FIG. 1, conventional laser scanning systems have the capacity to scan and read bar code symbols 116 over the laser scanning field 115 using a laser scanning beam 10 generated from a conventional visible laser diode (VLD), an aperture stop, and a collimating lens.
As shown in FIG. 2 attempts to improve the depth of focus of conventional laser scanning systems have proposed the use of aperture stops and axicon lens combinations, after the light collimating lens, to focus the laser beam and minimize beam spread during the propagation of the laser beam 10 during scanning operations within the working range of the system.
U.S. Pat. No. 5,080,456 to Katz et al and U.S. Pat. No. 5,331,143 to Marom et al disclose the use of axicon (i.e. rotationally-symmetric) optical elements for shaping laser beams, and extending the depth of focus thereof in laser scanning systems.
While axicon elements help to extend the depth of focus of laser beams, the resulting laser beams typically have limitations when scanning fine (i.e. 3 mil) code symbols at short ranges, while allowing reading of coarse (i.e. 100 mil) code symbols at long ranges (greater then 100 inches from the light transmission window).
Thus, there is a great need in the art to provide an improved method of and apparatus for generating laser beams having an extended depth of focus, without the shortcoming and drawbacks of prior art systems which, hitherto, have compromised system performance in significant ways.